The present invention relates to current measurement, and, in particular, to an improved current measuring circuit which retains the polarity information of a signal representative of the current being measured after full wave rectification of the signal.
Current measuring circuits have many applications, and in particular, one such application is in a trip unit for a circuit breaker. U.S. Pat. No. 4,631,625, issued on Dec. 23, 1986, discloses a microprocessor based trip unit for a circuit breaker within which a current measuring device is incorporated. In FIG. 1 of U.S. Pat. No. 4,631,625 the current measuring device is generally illustrated as including current transformers 30-33, a rectifier 34, signal converters 34 and an analog invertor 37. The microcomputer of U.S. Pat. No. 4,631,625 ultimately utilizes the signals from the analog invertor 37 to calculate a ground fault current.
Some prior art ground fault protection schemes modify signals representative of the current being measured in such a way that the polarity information of these signals is not preserved. An RMS analysis of these signals can then be performed without the polarity information to calculate a ground fault current.